87. Leptophilic dark matter

Recall the tempest in the physics blogosphere during the last two weeks – I mean, the one due to the CDF anomaly? As Jester at RESONAANCES said, whether we want it or not, it will essentially dominate model building for the next few months, and another confirmation to this statement is the recent paper by Erich Poppitz and Patrick Fox.

The authors introduce a model somewhat similar to the one Nima Arkani-Hamed with collaborators has recently proposed (to be brief, I shall further call it Nima’s model). While in Nima’s model dark matter carries charges of the non-abelian gauge group, and the corresponding symmetry is broken at the scale of 1 GeV, in Erich/Patrick model dark matter and leptons are charged over abelian gauge symmetry.

As well as Nima’s model, Erich/Patrick model is supposed to explain the excess (in comparison to cosmic rays) in total flux of electrons+positions that is peaked at GeV recently observed by PAMELA and ATIC. Both papers claim that the excess is due to the decay of dark matter particles.

It seems that DM explanation works fine if dark matter first annihilates with production of some intermediate state lighter than proton, and the latter then annihilates into electrons and positrons (production of hardons is suppressed since the intermediate state is too light). If one does not want to put strong constraints on the mass of the intermediate state, one can say that it is interacting at the tree level only with leptons, but not with quarks or gauge bosons. That is the idea Fox and Poppitz use in their paper, and that is why the title of the paper is “Leptophilic dark matter”.

The model they propose is the following: the dark matter (Dirac fermion) is charged under internal gauge symmetry broken by the scalar Higgs field. The coupling between Standard Model particles and  dark matter particles is through gauge boson. In the SM sector, the latter also couples (only) to two generations of leptons. The dark matter has a mass of the order 500-800 GeV, and mass of the gauge boson is less than 50 GeV.

So, why did not Arkani-Hamed et al. choose this simpler scenario and wanted DM to be charged under a non-abelian gauge symmetry instead? The answer to this question, as I understand from their paper, is that the symmetry does not allow to generate a flux of electrons and positrons suffcient to explain the INTREGRAL signal (the experiment Erich and Patrick do not discuss).